Inorg. Chem. 2010, 49, 9777–9787 9777 DOI: 10.1021/ic1006296
Structural Distortions upon Oxidation in Heteroleptic [Cp2W(dmit)] Tungsten Dithiolene Complex: Combined Structural, Spectroscopic, and Magnetic Studies ,‡ Eric W. Reinheimer,† Iwona Olejniczak,‡ Andrzej Łapinski,‡ Roman Swietlik,* Olivier Jeannin,† and ,† Marc Fourmigue* †
Sciences Chimiques de Rennes, Universite de Rennes I & CNRS UMR 6226, Campus de Beaulieu, 35042 Rennes, France, and ‡Institute of Molecular Physics, Polish Academy of Sciences, ul. M. Smoluchowskiego 17, 60-179 Pozna n, Poland
Received April 6, 2010
Four different cation radical salts are obtained upon electrocrystallization of [Cp2W(dmit)] (dmit = 1,3-dithiole-2-thione4,5-dithiolato) in the presence of the BF4-, PF6-, Br-, and [Au(CN)2]- anions. In these formally d1 cations, the WS2C2 metallacycle is folded along the S 3 3 3 S hinge to different extents in the four salts, an illustration of the noninnocent character of the dithiolate ligand. Structural characteristics and the charge distribution on atoms, for neutral and ionized complexes with various folding angles, were calculated using DFT methods, together with the normal vibrational modes and theoretical Raman spectra. Raman spectra of neutral complex [Cp2W(dmit)] and its salts formed with BF4-, AsF6-, PF6-, Br-, and [Au(CN)2]- anions were measured using the red excitation (λ = 632.8 nm). A correlation between the folding angle of the metallacycle and the Raman spectroscopic properties is analyzed. The bands attributed to the CdC and C-S stretching modes shift toward higher and lower frequencies by about 0.3-0.4 cm-1 deg-1, respectively. The solid state structural and magnetic properties of the three salts are analyzed and compared with those of the corresponding molybdenum complexes. Temperature dependence of the magnetic susceptibility shows the presence of one-dimensional antiferromagnetic interactions in the BF4-, PF6-, and [Au(CN)2]- salts, while an antiferromagnetic ground state is identified in the Br- salt below TNeel = 7 K. Interactions are systematically weaker in the tungsten salts than in the isostructural molybdenum analogs, a consequence of the decreased spin density on the dithiolene ligand in the tungsten complexes.
Introduction Heteroleptic complexes associating cyclopentadienyl and dithiolene ligands, such as Cp2Mo(dithiolene) complexes, are currently investigated, among other dithiolene complexes, as model compounds for molybdeno-enzymes.1 Indeed, the enzyme function is directly correlated with the different redox states these complexes can adopt, from MoIV to MoVI, through the intermediate radical, formally the d1 MoV species. The model Cp2Mo(dithiolene) complexes represent in that respect a useful tool, as several formally d1 MoV radical cations species were isolated and structurally investigated.2-4 Of particular interest in these d1 [Cp2Mo(dithiolene)]þ• complexes is the variable folding of the MoS2C2 metallacycle along the S 3 3 3 S hinge (θ), which is associated with a stabilization of the *To whom correspondence should be addressed. E-mail: swietlik@
[email protected] (M.F.). ifmpan.poznan.pl (R.S.), (1) McMaster, J.; Turner, J. M.; Garner, C. D. Prog. Inorg. Chem. 2004, 52, 539. (2) (a) Fourmigue, M.; Domercq, B.; Jourdain, I. V.; Molinie, P.; Guyon, F.; Amaudrut, J. Chem.—Eur. J. 1998, 4, 1714. (b) Fourmigue, M.; Lenoir, C.; Coulon, C.; Guyon, F.; Amaudrut, J. Inorg. Chem. 1995, 34, 4979. (3) (a) Clerac, R.; Fourmigue, M.; Gaultier, J.; Barrans, Y.; Albouy, P. A.; Coulon, C. Eur. Phys. J. B 1999, 9, 431. (b) Clerac, R.; Fourmigue, M.; Coulon, C. J. Solid State Chem. 2001, 159, 413. (4) Domercq, B.; Coulon, C.; Fourmigue, M. Inorg. Chem. 2001, 40, 371.
r 2010 American Chemical Society
oxidized form through a mixing of the Cp2Mo d-type fragment orbital and the dithiolene π orbital. Accordingly, the d0 complexes described with titanium for example are strongly folded (θ > 40) while the d2 Mo or W complexes are not (0 e θ e 10).5 Interestingly, a variety of folding angles (0 e θ e 30) was found for a given d1 complex depending on the nature of the counterion and associated solid-state structure.6 This folding, rationalized 20 years ago by Lauher and uckel calculations, has Hoffmann7 on the basis of extended H€ been also investigated experimentally in detail by photoelectron spectroscopy (PES)8,9 or EPR techniques.10,11 (5) Fourmigue, M. Coord. Chem. Rev. 1998, 178-180, 823. (6) Fourmigue, M. Acc. Chem. Res. 2004, 37, 179. (7) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729. (8) (a) Joshi, H. K.; Cooney, J. J. A.; Inscore, F. E.; Gruhn, N. E.; Lichtenberger, D. L.; Enemark, J. H. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 3719. (b) Cooney, J. J. A.; Cranswick, M. A.; Gruhn, N. E.; Joshi, H. K.; Enemark, J. H. Inorg. Chem. 2004, 43, 8110. (9) Cranswick, M. A.; Dawson, A.; Cooney, J. J. A.; Gruhn, N. E.; Lichtenberger, D. L.; Enemark, J. H. Inorg. Chem. 2007, 46, 10639. (10) Enemark, J. H.; Cooney, J. J. A.; Wang, J.-J.; Holm, R. H. Chem. Rev. 2004, 104, 1175. (11) (a) Drew, S. C.; Young, C. G.; Hanson, G. R. Inorg. Chem. 2007, 46, 2388. (b) Drew, S. C.; Hill, J. P.; Lane, I.; Hanson, G. R.; Gable, R. W.; Young, C. G. Inorg. Chem. 2007, 46, 2373. (c) Drew, S. C.; Hanson, G. R. Inorg. Chem. 2009, 48, 2224.
Published on Web 09/30/2010
pubs.acs.org/IC
9778 Inorganic Chemistry, Vol. 49, No. 21, 2010 Scheme 1. Origin of the Folding of the MS2C2 Metallacycle in Cp2M(dithiolene) Complexes, Illustrated Here on W d1 Complexesa
a No folding is found in d2 complexes (4 e-/2 orbitals). Systematic strong folding is found in d0 species (2e-/2 orbitals), variable folding in the intermediate formally d1 complexes.
As shown in Scheme 1b, the SOMO of such radical complexes is then the antibonding combination of the Cp2W d fragment orbital and the π-type dithiolene frontier orbital. Within this simple model, the interaction is strongest when the two fragment orbitals are closest in energy, giving rise to a SOMO essentially equally distributed on both fragments, hence the description of a formal d1 electron count since part of the SOMO is also delocalized on the dithiolene ligand. The relative ordering of the two frontier orbitals plays an important role. Indeed, if now an electron-rich dithiolene ligand such as dmit is used, its π-type frontier orbital is higher in energy, and the SOMO has therefore stronger ligand character (and less metal character).4 In those rare situations where the radical complex is not folded at all (Scheme 1a, θ=0), the two fragment orbitals are orthogonal and do not mix; the SOMO is then the highest of the two fragment orbitals. With an electron-rich dithiolate such as dmit, the unfolded radical could then be essentially localized on the dithiolene moiety.3 In this case, the unfolded complexes can essentially be described as a d2 Cp2W moiety interacting with a radical dithiolene ligand, and the formal d1 electron count description becomes a misuse of language. IR-Raman spectroscopy provides another useful tool to investigate this folding in the oxidized species, as it is anticipated that an unfolded complex would concentrate most of the charge density on the dithiolene moiety, with associated important C-S shortening and CdC bond lengthening effects. On the other hand, a more folded complex with a larger spin density on the metal should exhibit weaker effects on the C-S and CdC, with their associated vibrational spectroscopy signatures. Actually, a clear-cut correlation in Mo complexes has been recently reported,12 between the folding angle of the MoS2C2 metallacycle and the wavenumber of characteristic Raman bands such as the CdC and C-S stretching vibrations where this effect is strongest. The folding of [Cp2Mo(dmit)]þ• cations was found to be related to a low-wavenumber shift (0.5 - 0.6 cm-1/deg) of Raman bands assigned to the CdC and some C-S stretching vibrations. From a materials science (12) Swietlik, R.; Łapinski, A.; Fourmigue, M.; Yakushi, K. J. Raman Spectrosc. 2009, 40, 2092–2098.
Reinheimer et al. point of view, these radical complexes are also of interest as the spin density distribution is directly dependent on the folding angle. With dithiolene ligands such as those used for the elaboration of square-planar conducting salts,13 that is, dmit2- (dmit: 1,3-dithiole-2-thione-4,5-dithiolato) or dddt2(5,6-dihydro-1,4-dithiine-2,3-dithiolato), the molybdenum complexes, in their radical form, [Cp2Mo(dmit)]þ• or [Cp2Mo(dddt)]þ•, were shown to adopt different folding angles in the solid state between 0 and 30, depending on the nature of the counterion, revealing a high flexibility with an associated variable spin density delocalization.14 By analogy with the extensive work performed on molybdenum dithiolene complexes as models of Mo enzymes,1 several mono-15,16 or bis(dithiolene) tungsten complexes17-19 have been also described to date as models20,21 for corresponding tungsten enzymes.22,23 The latter are particularly interesting as they are essentially observed in thermophilic or hyperthermophilic microorganisms, by contrast with the molybdenum enzymes found in mesophilic ones. Several explanations have been proposed for the actual preference for one or another metal at the active sites of the enzymes,24,25 such as for example stronger π-π interactions between sulfur and metal atoms in the tungsten complexes.25b Differences of redox potentials of the Mo and W OdM(dithiolene)22-/1complexes were also addressed.26 Recently, Schulzke demonstrated that the electrochemical properties of several pairs of Mo and W complexes showed huge differences in the response of their redox potential to temperature changes.27 The higher (13) Kato, R. Chem. Rev. 2004, 104, 5319. (14) Fourmigue, M. Top. Organomet. Chem. 2009, 27, 161. (15) (a) Wang, J.-J.; Groysman, S.; Lee, S. C.; Holm, R. H. J. Am. Chem. Soc. 2007, 129, 7512. (b) Groysman, S.; Wang, J.-J.; Tagore, R.; Lee, S. C.; Holm, R. H. J. Am. Chem. Soc. 2008, 130, 12794. (c) Groysman, S.; Majumdar, A.; Zheng, S.-L.; Holm, R. H. Inorg. Chem. 2010, 49, 1082. (16) (a) Sproules, S. A.; Morgan, H. T.; Doonan, C. J.; White, J. M.; Young, C. G. Dalton Trans. 2005, 3552. (b) Eagle, A. A.; George, G. N.; Tiekink, E. R. T.; Young, C. G. J. Inorg. Biochem. 1999, 76, 39. (c) Eagle, A. A.; Harben, S. M.; Tiekink, E. R. T.; Young, C. G. J. Am. Chem. Soc. 1994, 116, 9749. (17) (a) Ueyama, N.; Oku, H.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 7310. (b) Stewart, L. J.; Bailey, S.; Bennett, B.; Charnock, J. M.; Garner, C. D.; McAlpine, A. S. J. Mol. Biol. 2000, 299, 593. (18) (a) Donahue, J. P.; Lorber, C.; Nordlander, E.; Holm, R. H. J. Am. Chem. Soc. 1998, 120, 3259. (b) Donahue, J. P.; Goldsmith, C. R.; Nadiminti, U.; Holm, R. H. J. Am. Chem. Soc. 1998, 120, 12869. (c) Lorber, C.; Donahue, J. P.; Goddard, C. A.; Nordlander, E.; Holm, R. H. J. Am. Chem. Soc. 1998, 120, 8102. (19) (a) Sung, K.-M.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1920. (b) Sung, K.-M.; Holm, R. H. Inorg. Chem. 2001, 40, 4518. (c) Goddard, C. A.; Holm, R. H. Inorg. Chem. 1999, 38, 5389. (d) Lim, B. S.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 1920. (20) (a) Das, S. K.; Biswas, D.; Maiti, R.; Sarkar, S. J. Am. Chem. Soc. 1996, 118, 1387. (b) Yadav, J.; Das, S. K.; Sarkar, S. J. Am. Chem. Soc. 1997, 119, 4315. (21) (a) Sugimoto, H.; Tajima, R.; Sakurai, T.; Ohi, H.; Miyake, H.; Itoh, S.; Tsukube, H. Angew. Chem., Int. Ed. 2006, 45, 3520. (b) Sugimoto, H.; Tano, H.; Toyota, K.; Tajima, R.; Miyake, H.; Takahashi, I.; Hirota, S.; Itoh, S. J. Am. Chem. Soc. 2010, 132, 8. (22) Stewart, L. J.; Bailey, S.; Bennet, B.; Charnock, J. M.; Garner, C. D.; McAlpine, A. S. J. Mol. Biol. 2000, 299, 593. (23) (a) Oku, H.; Ueyama, N.; Kondo, M.; Nakamura, A. Inorg. Chem. 1994, 33, 209. (b) Helton, M. E.; Gebhart, N. L.; Davis, E. S.; McMaster, J.; Garner, C. D.; Kirk, M. L. J. Am. Chem. Soc. 2001, 123, 10389. (24) de Bok, F. A. M.; Hagedoorn, P.-L.; Silva, P. J.; Hagen, W. R.; Schiltz, E.; Fritsche, K.; Stams, A. J. M. Eur. J. Biochem. 2003, 270, 2476. (25) (a) Hochheimer, A.; Linder, D.; Thauer, R. K.; Hedderich, R. Eur. J. Biochem. 1995, 234, 910. (b) Johnson, M. K.; Rees, D. C.; Adams, M. W. W. Chem. Rev. 1996, 96, 2817. (26) Tucci, G. C.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 1998, 37, 1602. (27) Schulzke, C. Dalton Trans. 2005, 713. (b) D€oring, A.; Schulzke, C. Dalton Trans. 2010, 39, 5623.
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temperature sensitivity of the redox properties of the W complexes was associated with more severe geometric changes upon oxidation which might increase the protein’s reorganization energies needed. If we consider the simple monodithiolene complexes formulated as Cp2W(dithiolene), a few neutral examples were described with benzenedithiolate (bdt) or toluenedithiolate (tdt) ligands,28 and more recently with the sulfur-rich dddt and dmit ligands.29 However, among them, only a very limited number of formally d1, oxidized species were described, limiting the identification of systematic trends, for comparison with the analogous Mo complexes. Indeed, the only two structurally characterized examples of d1 W complexes, that is, [Cp2W(dmit)]þ in its AsF6- and TCNQF4- salts, exhibit very different geometries with folding angles of 0 and 27.56(1), respectively.4,2a In order to establish whether such tungsten complexes are indeed more prone to larger geometrical modifications upon oxidation, as suggested by earlier electrochemical27 and theoretical studies,4 we decided to expand the family of such open-shell d1 tungsten derivatives and prepared four novel salts of the [Cp2W(dmit)]þ• radical cation with, respectively, the BF4-, the PF6-, the Br-, and the [Au(CN)2]- anions. The geometry adopted by the complexes in the four crystalline salts, and particularly the different folding angles of the WS2C2 metallacycle, have been correlated to their vibrational spectra, through the specific evolution of the C-S and CdC Raman signatures. Furthermore, the solid state structures adopted by the four salts have been analyzed in detail to rationalize the observed magnetic behaviors investigated by the temperature dependence of the magnetic susceptibility. This extended set of radical tungsten complexes allows now for interesting comparisons, within the series as well as with the analogous molybdenum complexes, as described below. Results and Discussion Molecular structures. [Cp2W(dmit)] was prepared from the reaction of Cp2WCl2 and (Naþ)2(dmit2-) as reported elsewhere.2,4 Its galvanostatic electrocrystallization in the presence of [n-Bu4N][BF4], [n-Bu4N][PF6], [PPh4][Br], or [n-Bu4N][Au(CN)2] afforded black crystals on the anode. It should be stressed indeed that this electrocrystallization technique30 is not limited to the crystal growth of conducting materials but is also well suited for the crystallization of insulating magnetic salts such as those described here, when the proper conditions for crystallization are met. Single crystal X-ray diffraction experiments were performed on the four salts. The BF4- and the PF6- salts are isostructural (monoclinic, space group P21/ c), with both the cation and anion in general position in the unit cell. The only difference between the two salts is a disorder which affects one Cp ring and the BF4- anion in [Cp2W(dmit)][BF4], while no disorder is found in the PF6- salt. The two other salts with Br- and [Au(CN)2]crystallize respectively in the monoclinic system, space (28) (a) Green, M. L. H.; Lindsell, W. E. J. Chem. Soc. 1967, 1455. (b) Debaerdemaeker, T.; Kutoglu, A. Acta Crystallogr., Sect. B 1973, 29, 2664. (c) Lindsell, W. E. J. Chem. Soc., Dalton Trans. 1975, 2548. (d) Klapoetke, T.; Koepf, H. Z. Anorg. Allg. Chem. 1988, 558, 217. (29) (a) Jourdain, I. V.; Fourmigue, M.; Guyon, F.; Amaudrut, J. J. Chem. Soc., Dalton Trans. 1998, 483. (b) Jourdain, I. V.; Fourmigue, M.; Guyon, F.; Amaudrut, J. Organometallics 1999, 18, 1834. (30) Batail, P.; Boubekeur, K.; Fourmigue, M.; Gabriel, J.-C. P. Chem. Mater. 1998, 10, 3005.
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Figure 1. View of the cation radical [Cp2W(dmit)]þ• in its PF6- salt. Note the folding of the WS2C2 metallacycle along the S1 3 3 3 S2 hinge. Ellipsoids are shown at the 50% probability level.
group P21/c, and the orthorhombic system, space group P212121. In the following, we will first describe the molecular geometry evolutions within these series together with their Raman spectroscopic properties, and we will discuss in a second part the solid state structures these radical species adopt in the solid state and the correlation with their magnetic behavior. As shown in Figure 1, the cation radical [Cp2W(dmit)]þ• species adopts a recurrent geometry with a characteristic distortion of the WS2C2 metallacycle through a folding along the S1 3 3 3 S2 hinge. The most pertinent geometrical features (bond distances and angles, folding angle) of the cation in the four salts are collected in Table 1, together with those of two salts reported earlier with TCNQF4 and with AsF6-. The [Cp2W(dmit)]þ• cation in the four salts exhibits a comparable folding angle of the metallacycle, 27.71(9) in the BF4- salt, 26.8(2) in the PF6- salt, 31.55(7) in the Br- salt, and 29.7(2) and 31.7(2) in the two crystallographically independent molecules in the [Au(CN)2]- salt. Comparison of the CdC and C-S bond lengths with those found earlier in the unfolded AsF6salt are particularly interesting (Table 1). Indeed, the folded complexes systematically exhibit a shorter CdC bond and longer C-S bonds, indicating that the oxidation affects the dithiolene ligand to a smaller extent in the folded complexes. An interesting comparison can be also made with the analogous molybdenum salts (Table 2). Indeed, the corresponding [Cp2Mo(dmit)]þ• salts have been reported with all except the [Au(CN)2]- anion. It appears that the salts with Mo and W are not systematically isostructural and that, even when they are, the folding angles are systematically larger in the tungsten than in the molybdenum complexes. This trend has been attributed to a stronger interaction between the Cp2M and dmit frontier orbitals when M = W.2a,4 DFT Calculations. Quantum chemical calculations for tungsten complexes were performed at the B3LYP/LanL2DZ theory level, as previously described for the molybdenum complexes.12 The geometry optimization of the neutral complex and its radical cation [Cp2W(dmit)]þ• gave an almost undistorted geometry (in both cases θ = 2.8). Upon complex ionization, the length of the metallacycle CdC bond increases while the C-S inner bonds connected with the CdC bond decrease their lengths (Table 3). Subsequently, we optimized the geometry of the [Cp2W(dmit)]þ• cation by fixing different folding angles (θ = 10, 20, 30). The bond analysis shows that on folding, all the C-S bonds grow, while the CdC and CdS bonds decrease in length. Such modifications are in agreement with the above-reported X-ray crystallographic data. Additionally, the average charge on the different atoms of the neutral complex and its cation with different folding
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Table 1. Important Bond Distances and Angles in the Different [Cp2W(dmit)]X Saltsa counter ion BF4 PF6
-
-
Br[Au(CN)2]
-
mol 1 mol 2
TCNQF4
-
AsF6a
M-S (A˚)
C-S (A˚)
CdC (A˚)
S-M-S (deg)
θ (deg)
ref
2.4136(15) 2.4317(16) 2.408(4) 2.422(6) 2.4330(9) 2.4343(9) 2.427(2) 2.423(3) 2.426(3) 2.425(2) 2.426(4) 2.441(4) 2.429(1)
1.718(3) 1.723(3) 1.709(9) 1.714(8) 1.725(3) 1.718(3) 1.719(8) 1.717(9) 1.718(9) 1.731(9) 1.721(6) 1.722(6) 1.693(5)
1.380(8)
83.13(5)
27.71(9)
this work
1.374(11)
82.94(7)
26.8(2)
this work
1.379(4)
82.98(3)
31.55(7)
this work
1.366(12)
82.89(9)
31.7(2)
this work
1.348(12)
83.56(8)
29.7(2)
1.374(8)
83.18(5)
27.56(1)
1.384(9)
83.86(7)
0
2a 4
The torsion angle θ represents the angle between the WS2 and S2C2 mean planes of the WS2C2 metallacycle.
Table 2. Folding Angles of the Metallacycle in Various Salts of [Cp2Mo(dmit)] and [Cp2W(dmit)] counter ion
θMo
refMo
θW
refW
Mo vs. W
BF4PF6BrTCNQF4AsF6-
23.21(5) 0 30.45(4) 10.2(1) 0
3b 3a 3b 2a 3a
27.71(9) 26.8(2) 31.55(7) 27.56(1) 0
this work this work this work 2a 4
isostructural different isostructural different isostructural
Table 3. Calculated Bond Lengths of Neutral and Ionized Cp2W(dmit) Complex for Various Folding Angles (θ), at the B3LYP/LanL2DZ Theory Level (Atom Labeling Is Given in Figure 1) radical cation compound folding angle
neutral
0
10
20
30
C1-S1 C2-S2 C1dC2 C1-S3 C2-S4 C3-S3 C3-S4 C3dS5
1.823 1.820 1.350 1.827 1.828 1.824 1.824 1.684
1.778 1.778 1.397 1.807 1.807 1.837 1.837 1.662
1.779 1.779 1.396 1.808 1.808 1.837 1.837 1.662
1.784 1.784 1.392 1.809 1.809 1.839 1.839 1.662
1.789 1.789 1.388 1.809 1.809 1.841 1.841 1.662
Table 4. Charge Distribution (Mulliken) on Atoms of Neutral and Ionized Cp2W(dmit) Complex with Various Folding Angles (θ) Calculated at the B3LYP/ LanL2DZ Theory Level (Atom Numbering Is Given in Figure 1) [Cp2W(dmit)]þ• cation atom
neutral complex
θ = 0
θ = 10
θ = 20
θ = 30
S1 S2 C1 C2 S3 S4 C3 S5 P (C3S5) W P (Cp2W)
0.162 0.127 -0.322 -0.313 0.385 0.352 -0.611 0.013 -0.207 -0.594 0.207
0.297 0.259 -0.317 -0.307 0.481 0.449 -0.609 0.189 0.442 -0.664 0.558
0.299 0.263 -0.309 -0.318 0.446 0.478 -0.609 0.185 0.435 -0.670 0.565
0.270 0.306 -0.316 -0.325 0.440 0.473 -0.611 0.178 0.415 -0.685 0.585
0.280 0.314 -0.331 -0.338 0.435 0.468 -0.612 0.171 0.387 -0.689 0.613
angles was evaluated (Table 4). Going from the neutral complex to the radical cation species, we first observe that a large part of positive charge is transferred to the dithiolene moiety, illustrating the important contribution of the dithiolene moiety in the oxidation. Furthermore, upon [Cp2W(dmit)]þ• cation folding, this average positive charge on dmit actually decreases by about 11%, when bending the cation from 0 to 30, showing that this
Figure 2. Theoretical Raman spectra within the CdC stretching for the [Cp2W(dmit)]þ• cation, calculated for different folding angles at the B3LYP/LanL2DZ theory level (non-normalized data).
dithiolene contribution decreases upon folding. While analogous modifications of the bond lengths upon complex folding were also observed in the case of the [Cp2Mo(dmit)]þ• cation, the charge decrease on the dmit ligand for the [Cp2Mo(dmit)]þ• cation upon bending from 0 to 30 was found to be much smaller (about 0.5%). We have also calculated the frequencies of vibrational modes and Raman scattering intensities for both neutral and ionized complexes with different folding angles. The mode description was done by visual inspection of each individual mode by using the GaussView program. For the neutral [Cp2W(dmit)] complex, one CdC stretching mode is seen, and it exhibits a strong low-frequency shift upon ionization (about 70 cm-1). For the [Cp2W(dmit)]þ• cation, we have found two vibrational modes related to the CdC stretching (Figure 2). The visual inspection of these modes shows that in the case of the radical cation, in addition to the CdC stretching, the Cp groups also perform vibrations which are different for each CdC mode. This contribution of the Cp groups to the CdC vibration modes of the [Cp2W(dmit)]þ• cation is not observed in the neutral complex. On cation folding from θ = 0 to θ = 30, the lower-frequency CdC mode shows nearly no shift, while the higher-frequency one shifts up by about 4 cm-1. Simultaneously, the related Raman bands change their theoretical intensities. It should be emphasized here
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Inorganic Chemistry, Vol. 49, No. 21, 2010
that for the [Cp2Mo(dmit)]þ• cation investigated earlier, only one CdC mode was found, and the related Raman band showed a low-frequency shift upon folding.12 The folding of the [Cp2W(dmit)]þ• cation by about θ = 30 also has an influence on other vibrational stretching modes: the CdS mode shifts up by 4 cm-1 and the C-S modes shift up or down depending on the mode, while the frequency W-S mode is nearly constant. Experimental Raman Spectra. Raman spectra of the [Cp2W(dmit)]X salts were measured on single crystals for the electrical vector parallel and perpendicular to the direction of the maximum CdC band intensity. The spectra of the various salts were similar. As an example, we show the spectra of the [Cp2W(dmit)]BF4 salt within the region of strongest vibrational features (Figure 3). The spectra of all the salts and also the neutral complex, for polarization corresponding to the maximum CdC band intensity, are displayed in Figure 4, and positions of the most important bands are listed in Table 6. While the DFT calculations of the [Cp2W(dmit)]þ• cation revealed the existence of two vibrational modes attributed to the CdC bond stretching, our analysis of the experimental data shows only one Raman line for all studied salts. Apparently, in crystals, these modes have almost the same frequencies, and they cannot be distinguished in our Raman experiments, at room temperature at least. On cation folding from θ = 0 (in AsF6- salt) to 27-32 (in the four other salts), the CdC band exhibits a shift of approximately 10 cm-1 toward higher frequencies (Table 5). This observation is in agreement with the tendency suggested by the theoretical calculations, which show that the high-frequency CdC mode increases its frequency and relative intensity. Consequently, the band, being a superposition of these two components, should shift toward
Figure 3. Raman spectra of the [Cp2W(dmit)]BF4 salt recorded for two different orientations of the electrical vector of the exciting laser beam (λ = 632.8 nm): parallel (E||) and perpendicular (E^) to the maximum intensity of the band assigned to the CdC stretching vibration.
9781
higher frequencies. It is important to note that the behavior of the CdC mode in the [Cp2W(dmit)]þ• cation is different from that of the [Cp2Mo(dmit)]þ• cation, where the folding actually decreased the frequency of the CdC mode.12 The most important changes induced by folding are observed for Raman bands related to C-S stretching at about 920 cm-1: they exhibit a low-frequency shift of about 10 cm-1, from the unfolded AsF6- salt to the strongly folded salts (θ = 27-32). A slightly larger (about 15 cm-1) frequency shift had been observed for the [Cp2Mo(dmit)]þ• cation.12 It therefore clearly appears that the C-S band at about 920 cm-1 is probably the best indicator of metallacycle distortion in both Mo and W complexes in the solid state. In conclusion, by comparison with the unfolded AsF6salt, the theoretical calculations and Raman data show that the strong folding observed in the W complexes described here is associated with a transfer of the oxidation site, from the dithiolene ligand to the Cp2W moiety. From a magnetic point of view, this should be associated with a decreased spin density on the dithiolene ligand. To address this point, we will therefore describe in the following the solid-state magnetic response of these four salts. Indeed, while a wide variety of structures and magnetic behaviors has been reported for analogous molybdenum complexes,6,14 very few examples involving tungsten complexes have been reported to date, with [Cp2W(dmit)] as mentioned above,2a,4 with [Cp2W(dmid)] (dmid: 1,3-dithiole-2-one-4,5-dithiolato),2a and with the
Figure 4. Raman spectra of the neutral Cp2W(dmit) complex and [Cp2W(dmit)]X salts (X = AsFh6, PFh6, BFh4, Brh , Au(CN)h2) obtained at room temperature using red excitation (λ = 632.8 nm). The electrical vector of the laser beam was parallel to the direction of the maximum CdC band intensity.
Table 5. Selected Raman Bands of the Neutral Cp2W(dmit) and Its Radical Cation in [Cp2W(dmit)]X Salts, X = AsF6, PF6, BF4, Br, and Au(CN)2, Recorded with Excitation λ = 632.8 nm compound folding angle (θ) Raman bands
neutral
AsF6-
0.0 1451 1329 1053 1042 891, 886 932 523 508 464 482 344 358, 346
PF6-
BF4-
Br-
Au(CN)2-
assignement
26.8(2) 1334 1028 922 513 476 364, 349
27.71(9) 1337 1022 922 513 477 365, 349
31.55(7) 1348 1024 920 516 475 366, 351
31.7(2), 29.7(2) 1339 1026 922 509 479 361, 350
CdC stretching CdS stretching C-S stretching C-S stretching (ring breathing) C-S stretching W-S stretching
9782 Inorganic Chemistry, Vol. 49, No. 21, 2010
Figure 5. Projection view along a of the unit cell of [Cp2W(dmit)]BF4. The BF4- anion is disordered on two positions.
Reinheimer et al.
Figure 7. Temperature dependence of the magnetic susceptibility of [Cp2W(dmit)]BF4 at 5000 G. The dotted and solid lines are the best fits obtained by using eqs 2 (R = 0.959) and 3 (R = 0.989), respectively (see text).
0.030 eV and the interaction along a with molecule (1 þ x, y, z) or (-1 þ x, y, z) amounts to 0.024 eV. The system can therefore be described as antiferromagnetic coupled dyads further interacting with neighboring molecules along a. The temperature dependence of the magnetic susceptibility (Figure 7) shows an activated regime at low temperatures with a susceptibility maximum around 100 K. Considering the structural description of this salt, the susceptibility data should be described with the theoretical expression for the magnetic susceptibility of an antiferromagnetically coupled S = 1/2 dimer with a singlet ground state, that is, the Bleaney-Bower equation, which reads per complex as
Figure 6. View of the chains of cation radical dyads running along a in [Cp2W(dmit)]BF4. The thick dotted lines indicate the strongest intermolecular interaction between molecules at (x, y, z) and (1 - x, -y, 2 - z).
diselenolene complex [Cp2W(dsit)] (dsit: 1,3-dithiole-2thione-4,5-diselenato).2a,4 The four novel salts reported here give an opportunity to expand these series, allowing now for pertinent comparisons, between the different W salts as well as with the analogous molybdenum ones, as detailed below. [Cp2W(dmit)]BF4. The [Cp2W(dmit)]BF4 salt crystallizes in the monoclinic system, space group P21/c, with both the cation and anion in general positions in the unit cell. A projection view along a (Figure 5) shows that the radical cations are organized into dyads with a face-toface σ overlap of the dmit moieties and short S 3 3 3 S contacts (3.50-3.57 A˚), while these dyads interact sideways along a with larger S 3 3 3 S contacts at 3.70 A˚. In order to evaluate the extent of interactions within the dyad and between dyads along a, calculations of the βHOMO-HOMO interactions energies were performed in the frame of extended H€ uckel tight binding methods.31,32 The intradimer interaction shown with thick dotted lines in Figure 6 between molecules (x, y, z) and (1 - x, -y, 2 - z) amounts to 0.086 eV, while the other face-to-face interaction with molecule (-x, -y, 2 - z) amounts to (31) Whangbo, M.-H.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 6093. (b) Hoffmann, R. J. Chem. Phys. 1963, 39, 13978. (32) Ren, J.; Liang, W.; Whangbo, M.-H. Crystal and Electronic Structure Analysis Using CAESAR; PrimeColor Software: http://www.primec.com/ (accessed Sep 2010), 1998.
χBB ¼
Ng2 β2 kB Tð3 þ expð-2J=kB TÞÞ
ð1Þ
A second contribution observed at the lowest temperatures is attributed to magnetic defects whose contribution follows a Curie-Weiss behavior. Considering these two contributions, the susceptibility was accordingly fitted (dotted line in Figure 7) with the following expression: χ ¼ F
Nβ2 þ ð1 - FÞχBB þ χdia kB ðT - θÞ
ð2Þ
where χdia is the diamagnetic contribution and F, the fraction of paramagnetic impurities. We observe that the fit is not satisfactory (correlation coefficient R = 0.959), with the best set of parameters found to be χdia = 4.7(1) 10-5 cm3 mol-1, F = 0.022. and J/kB = -85(2) K. In order to perform a better description of the systems, the weaker interactions between dyads were taken into consideration in the model by the mean-field approximation,33 with χ ¼ F
Nβ2 χBB þ ð1 - FÞ þ χdia 0 kB ðT - θÞ 1 - ð4zJ χBB =Ng2 β2 Þ ð3Þ
(33) (a) Myers, B. E.; Berger, L.; Friedberg, S. J. Appl. Phys. 1969, 40, 1149. (b) Marsh, W. E.; Patel, K. C.; Hatfield, W. E.; Hodgson, D. J. Inorg. Chem. 1982, 22, 511.
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Figure 9. Projection view along b of the unit cell of [Cp2W(dmit)]Br.
Figure 8. Temperature dependence of the magnetic susceptibility of [Cp2W(dmit)]PF6 at 5000 G. The solid line is the best fit obtained by using eq 3 (R = 0.982).
where z is the number of nearest next neighbors (z = 3) and J0 is the average of interdimer magnetic exchange constants. A much improved agreement with the experimental data was obtained with this fit (full line in Figure 7), with the following set of parameters: χdia = 2.2(1) 10-5 cm3 mol-1, F = 0.02, JW/kB = -82(1) K, and JW0 /kB = -7.8(4) K. It should be stressed here that very similar behavior was observed in the isostructural molybdenum salt, [Cp2Mo(dmit)](BF4),3b albeit the interactions were notably stronger in the latter (JMo/kB = -145 K, JMo0 /kB = -15 K). This trend has been already noticed in other salts known with both Mo and W metal centers.34 It finds its origin in the stronger interaction between the Cp2W and dmit fragment orbitals, which are closer in energy than in the Cp2Mo/dmit situation. As a consequence, the folding angles are often larger in the tungsten complexes, and the HOMO of the cation radical species, which is the antibonding combination of both fragment orbitals, has a stronger metal character in the W complex and hence lesser dithiolene character. Since the dominant intermolecular interactions are actually based on the dmit/dmit interactions, these are decreased in the tungsten salts, when compared with isostructural molybdenum analogs. [Cp2W(dmit)]PF6. As the BF4- and PF6- salts are isostructural, they exhibit an identical solid state organization (Figure 5), with radical cations associated into inversioncentered dyads with face-to-face interactions between the dmit moieties, with these dyads further interacting sideways along a. The temperature dependence of the magnetic susceptibility (Figure 8) exhibits a very similar behavior, without any field dependence, and with a susceptibility maximum observed at a lower temperature, which is 45 K vs 115 K in the BF4- salt. This clearly indicates that the antiferromagnetic interactions in the PF6- salt are strongly decreased, as also evidenced from the value of the J exchange constant determined from the same fit as above (eq 3). We found indeed the following set of parameters: χdia = 4.4 (1) 10-4 cm3 mol-1, F = 0.036, JW/kB = -34.9(6) K, JW0 /kB = -3.6(2) K, with therefore an intradyad exchange constant JW/kB around -35 K, to be compared with -82 K in the BF4- salt. These decreased interactions reflect a negative pressure effect attributable to the larger size of the PF6- anion when compared with (34) (a) Fourmigue, M.; Coulon, C. Adv. Mater. 1994, 6, 948. (b) Domercq, B.; Coulon, C.; Feneyrou, P.; Dentan, V.; Robin, P.; Fourmigue, M. Adv. Funct. Mater. 2002, 12, 359.
Figure 10. Detail of the hydrogen bond pattern around the Br- anion in [Cp2W(dmit)]Br. Table 6. C-H 3 3 3 Br Hydrogen Bond Characteristics in [Cp2W(dmit)]Br interaction Br 3 Br 3 Br 3 Br 3
3 3 3 3
3 H8 3 H11 3 H10 3 H9
(C-)H 3 3 3 Br/A˚
C(-H) 3 3 3 Br/A˚
C-H 3 3 3 Br/deg
2.823 2.899 2.925 3.013
3.72(5) 3.803(5) 3.817(7) 3.780(5)
163.4(2) 164.2(3) 161.3(3) 140.8(3)
the BF4- one. Indeed, albeit isostructural, the two salts have different unit cell volumes, 1721.8(7) A˚3 in the BF4salt and 1818.9(3) A˚3 in the PF6- salt. As a consequence, the radical cations are pushed apart from each other in the PF6- salt; the shortest intermolecular S 3 3 3 S contacts exceed 3.67 A˚ while they were found at 3.50 A˚ in the BF4- salt. Calculations of the βHOMO-HOMO interaction energies confirm this assumption, with the calculated intradyad interaction between (x, y, z) and (1 - x, -y, 2 - z) (see Figure 6) found at 0.059 eV, to be compared to 0.086 eV in the BF4- salt.35 If these values could at first sight appear relatively close to each other, one should recall that, in a Hubbard model,36 with U describing the on-site electronic repulsion and β the resonance integral, the exchange integral J between two adjacent radicals is written as β2/U. Thus, we expect the ratio J(BF4)/J(PF6) to behave approximately as [β(BF4)/β(PF6)]2. We find indeed J(BF4)/J(PF6) = 2.3 and [β(BF4)/β(PF6)]2 = 2.1. Note also that a similar, isostructural but even more expanded structure was reported earlier in the diselenolene analog with the larger AsF6- anion, that is, [Cp2W(dsit)][AsF6], where the cell volume amounted to 1932.6(3) A˚3.4 In this salt, sizable antiferromagnetic interactions were reported with a susceptibility maximum at 78 K, indicating that the cell expansion effect was counterbalanced by the stronger overlap offered by the selenium atoms. [Cp2W(dmit)]Br. The structure of the bromide salt, [Cp2W(dmit)]Br, is completely different; it crystallizes in (35) The two others interactions (x, y, z)/(-x, 2 - y, 1 - z) and (x, y, z)/ (1 þ x, y, z) are found at 0.025 and 0.036 eV, respectively. (36) Whangbo, M.-H. Acc. Chem. Res. 1983, 16, 95.
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Table 7. Calculated βHOMO-HOMO Interactions Energies between One Complex at the (x, y, z) Position and Its Neighbors in [Cp2W(dmit)]Br complex at
interaction
β (meV)
(x, 1.5 - y, -0.5 þ z) and (x, 1.5 - y, 0.5 þ z) (-x, 1 - y, 1 - z) (-x, 2 - y, 1 - z) (-x, -0.5 þ y, 1.5 - z) and (-x, 0.5 þ y, 1.5 - z) (x, 1 þ y, z) and (x, -1 þ y, z) (1 - x, 0.5 þ y, 1.5 - z) and (1 - x, -0.5 þ y, 1.5 - z)
S3 3 3S S3 3 3S S3 3 3S S3 3 3S Cp 3 3 3 dmit Cp 3 3 3 Cp
37.4 6.4 9.7 21.6 5.2 11.4
Figure 12. Temperature dependence of the susceptibility of [Cp2W(dmit)]Br at 5 kG. Inset: Field dependence of the susceptibility below 20 K.
Figure 11. Details of the intermolecular interactions between one [Cp2W(dmit)] complex at (x, y, z) and neighboring complexes in [Cp2W(dmit)]Br (see also Table 8).
the monoclinic system, space group P21/c, with both the cation and anion in a general position in the unit cell. As shown in Figure 9, the cations are associated into layers perpendicular to a, with the bromide anions hydrogen bonded to the organometallic cations through C-H 3 3 3 Br- interactions with the hydrogen atoms of the cyclopentadienyl rings (Figure 10 and Table 6). A complex set of intermolecular interactions characterizes the structure of the Br- salt as a given cation radical exhibits short intermolecular contacts with 10 neighboring molecules (Figure 11). As for the BF4- salt, calculations of the βHOMO-HOMO interaction energies were performed in the frame of extended H€ uckel tight binding methods, and the results are collected in Table 7. One observes that the stronger interactions develop within the bc plane through S 3 3 3 S and Cp 3 3 3 dmit interactions (along b) while these planes are connected to each other through Cp 3 3 3 Cp interactions.37 This gives rise to a threedimensional interaction network, as the interlayer Cp 3 3 3 Cp interaction is not negligible. The temperature dependence of the magnetic susceptibility of [Cp2W(dmit)]Br is shown in Figure 12. It exhibits (37) Such intermolecular Cp 3 3 3 Cp interactions have been also observed, to a much stronger extent, in other heteroleptic complexes such as [CpNi(dithiolene)]. See: Fourmigue, M.; Cauchy, T.; Nomura, M. CrystEngComm. 2009, 11, 1491 and references therein.
Figure 13. Projection view along a of the unit cell of [Cp2W(dmit)][Au(CN)2] 3 (H2O)0.5. Table 8. Calculated βHOMO-HOMO Interaction Energies between the Two Crystallographically Independent Complexes (Noted W1 and W2) and Their Neighbors within the Tetrameric Columns in [Cp2W(dmit)][Au(CN)2] interacting complexes
interaction
β (meV)
W1(x, y, z) vs W1(1 þ x, y, z) W2(x, y, z) vs W2(1 þ x, y, z) W1(x, y, z) vs W1(-0.5 þ x, 1.5 - y, 1 - z) W1(x, y, z) vs W2(x, y, z) W1(x, y, z) vs W2(1.5 þ x, 1.5 - y, 1 - z) W1(x, y, z) vs W2(0.5 þ x, 1.5 - y, 1 - z)
Cp/dmit Cp/dmit S3 3 3S S3 3 3S S3 3 3S S3 3 3S
28.6 51.1 34.6 12.1 16.3 4.8
a maximum at 10.6 K, confirming the presence of weak intermolecular antiferromagnetic interactions, while a fit to the Curie-Weiss law for T > 20 K gives a CurieWeiss temperature θW = -14.5(1) K. At the lower temperatures, the magnetic susceptibility exhibits a field dependence (Figure 12 inset) indicative of the presence of an antiferromagnetic ground state below TNeel(W) = 7 K.
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Table 9. Crystallographic Data of the [Cp2W(dmit)][X] Salts X-
BF4-
PF6-
Br-
[Au(CN)2]-
formula formula mass cryst syst space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 Z dcalc/g cm-3 μ/mm-1 data collected ind. data Rint obs. data (I > 2σ(I)) params refined R(F) wR(F2) residual d/e A˚-3
C13H10BF4S5W 597.17 monoclinic P21/c 6.5630(2) 15.3090(3) 17.1500(4) 90.0 92.200(3) 90.0 1721.8(7) 4 2.304 7.348 23978 3948 0.0297 3259 191 0.0282 0.0790 1.538, -0.917
C13H10F6PS5W 655.33 monoclinic P21/c 6.6514(7) 15.680(1) 17.442(2) 90.0 90.861(13) 90.0 1818.9(3) 4 2.393 7.067 17078 4347 0.1046 3113 235 0.0462 0.1109 2.944, -1.269
C13H10BrS5W 590.27 monoclinic P21/c 16.705(3) 7.279(1) 12.856(3) 90.0 96.26(3) 90.0 1553.8(5) 4 2.523 10.662 21364 3562 0.0325 3135 181 0.0178 0.0355 0.412, -0.787
C30H20Au2N4OS10W2 1534.82 orthorhombic P212121 7.5820(5) 16.438(3) 30.473(4) 90.0 90.0 90.0 3798.0(9) 4 2.684 14.322 34983 8671 0.0632 6118 443 0.0416 0.0690 1.642, -0.718
This behavior is fully coherent with the analysis of a threedimensional set of intermolecular interactions reported above and also follows the behavior of the isostructural molybdenum salt3b [Cp2Mo(dmit)]Br where θMo = -20 K with a similar antiferromagnetic ground state below TNeel(Mo) = 4.5 K. [Cp2W(dmit)][Au(CN)2]. The solid state structure of the [Au(CN)2]- salt is more complex than that of the three other BF4-, PF6-, and Br- salts described above. Indeed, [Cp2W(dmit)][Au(CN)2] 3 (H2O)0.5 (Figure 13) crystallizes in the orthorhombic system, space group P212121, with two crystallographically independent [Cp2W(dmit)]þ• cations as well as two crystallographically independent [Au(CN)2]- anions and one water molecule, hydrogen bonded to the nitrogen atom of a cyanide moiety with a O 3 3 3 N distance of 2.84(2) A˚ and a O 3 3 3 NtC angle of 157(1). The shortest Au 3 3 3 Au distance amounts to 3.28 A˚, with a C-Au 3 3 3 Au0 -C0 torsion angle of 85, compatible with the presence of an aurophilic interaction. Such aurophilic interactions are often found with gold(I) species such as [Au(CN)2]-, with distances as short as 3.10 A˚.38,39 The longer distance found here is to be correlated also with the C-Au 3 3 3 Au0 -C0 torsion angle. Indeed, theoretical studies and experimental results have shown that the Au 3 3 3 Au distances decrease with an increase in the torsion angle between adjacent [Au(CN)2]- units.40 The projection of (38) (a) Suarez-Varela, J.; Mota, A. J.; Aouryaghal, H.; Cano, J.; Rodriguez-Dieguez, A.; Luneau, D.; Colacio, E. Inorg. Chem. 2008, 47, 8143. (b) Suarez-Varela, J.; Sakiyama, H.; Cano, J.; Colacio, E. Dalton Trans. 2007, 249. (39) (a) Leznoff, D. B.; Lefebvre, J. Gold Bull. 2005, 38, 47 and references therein. (b) Zhou, H.-B.; Wang, S.-P.; Dong, W.; Liu, Z.-Q.; Wang, Q.-L.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P.; Cheng, P. Inorg. Chem. 2004, 43, 4552. (c) Colacio, E.; Lloret, F.; Kivek€as, R.; Suarez-Varela, J.; Sundberg, M. R.; Uggla, R. Inorg. Chem. 2003, 42, 560. (d) Lefebvre, J.; Callaghan, F.; Katz, M. J.; Sonier, J. E.; Leznoff, D. B. Chem.—Eur. J. 2006, 12, 6248. (e) Lefebvre, J.; Chartrand, D.; Leznoff, D. B. Polyhedron 2007, 26, 2189. (f) Katz, M. J.; Kaluarachchi, H.; Batchelor, R. J.; Bokov, A. A.; Ye, Z.-G.; Leznoff, D. B. Angew. Chem., Int. Ed. 2007, 46, 8804. (40) (a) Colacio, E.; Lloret, F.; Kivek€as, R.; Suarez-Varela, J.; Sundberg, M. R. Chem. Commum. 2002, 592. (b) Assefa, Z.; Omary, M. A.; McBurnett, B. G.; Mohamed, A. A.; Patterson, H. H.; Staples, R. J., Jr. Inorg. Chem. 2002, 41, 6274.
Figure 14. Temperature dependence of the magnetic susceptibility of [Cp2W(dmit)][Au(CN)2] at 5000 G. The solid line is the best fit obtained by using eq 3, R = 0.99947.
the unit cell along a (Figure 13) shows that the radical cations are organized into columns running along a, associated four-by-four, and isolated from other 4-fold columns by the [Au(CN)2]- and water molecules. Within these tetrameric columns, βHOMO-HOMO interactions energies were calculated and collected in Table 8. We found sizable interactions along the columns through Cp/dmit overlap combined with lateral interactions in between the columns through short S 3 3 3 S intermolecular contacts. This complex set of interactions cannot be summarized into a simple model such as a dimer, spin chain, or spin ladder. The temperature dependence of the magnetic susceptibility (Figure 14) is characterized with a susceptibility maximum at a low temperature (7.5 K), without any field dependence, an indication of the absence of an ordered three-dimensional ground state, in accordance with the essentially isolated tetrameric columns. On the basis of the calculated antiferromagnetic interactions within dyads (Table 8), a fit of the data was obtained from eq 3 involving antiferromagnetically coupled S = 1/2 dimers with a singlet ground state, with added contributions of a Curie tail at lower temperatures and of weaker interactions between dimers, giving F = 0.062(5), J/kB = -7.26(7) K, and J0 /kB = -2.19(1) K with z = 6.
9786 Inorganic Chemistry, Vol. 49, No. 21, 2010 Conclusions Several features have been identified here from the combination of spectroscopic, structural, and magnetic investigations performed on four novel [Cp2W(dmit)]þ• cation radical salts. Comparison with analogous Mo salts shows that a recurrent trend toward more folded metallacycle geometry is observed with most W complexes, a consequence of stronger interactions between the CpW and dithiolene fragment orbitals. This is also associated with a smaller electronic contribution of the dithiolene fragment in the SOMO, as evidenced from the calculated charge distribution, and the smaller frequency shift upon folding exhibited by Raman-active C-S bands (-10 cm-1 vs -15 cm-1 in the W vs the Mo complex when going from θ = 0 to θ = 30). In that respect, the unfolded structure of the AsF6- salt appears as an anomaly, and it is anticipated that this structure actually distorts upon cooling, as already observed in the unfolded PF6- and AsF6- salts of the Mo analog, Cp2Mo(dmit).3a The magnetic behavior of the different salts also demonstrates that the spin density on the dithiolene moiety is strongly decreased in the W complexes, when compared with analogous Mo complexes, as the antiferromagnetic interactions which characterize these systems, are systematically weaker with the former, whatever the dimensionality of these interactions, 1D or 3D. Experimental Section The neutral [Cp2W(dmit)] and [Cp2Mo(dmit)] were prepared according to published procedures.2,4 As electrolytes used in the electrocrystallization experiments, [n-Bu4N][BF4], [n-Bu4N][PF6], and [PPh4][Br] were commercially available; [n-Bu4N][Au(CN)2] was prepared from the commercially available potassium salt KAu(CN)2. Elemental analyses were performed at the Service de Microanalyse, ICSN, Gif/Yvette, France. Electrocrystallization Experiments. General. All electrocrystallization experiments were performed in a galvanostatic mode with two-compartment cells and platinum wire electrodes (2-cm-long, 1 mm in diameter) under thermostatted conditions. Details on the solvent, concentration, temperature, and current used are given below for each system. [Cp2W(dmit)][BF4]. The 1:1 salt was grown using the electrocrystallization method involving constant current oxidation of Cp2W(dmit) (15 mg) in the presence of electrolytic solutions prepared by dissolving [n-Bu4N][BF4] (0.16 g, 0.5 mmol) in 10 mL of freshly distilled CH2Cl2 to yield an electrolyte solution with a final concentration of 0.5 M. Single crystals were grown at a constant current density of 5 μA at a temperature of 5 C. Black block-like crystals were harvested from the anodic compartment of the electrochemical cell after a period of one week and washed with small amounts of freshly distilled CH2Cl2. Elem Anal. Calcd for C13H10BF4S5W (MW = 597.2070 g/mol): C, 26.15%; H, 1.69%. Found: C, 25.93; H, 1.68. IR (KBr, cm-1) 3088, 3132, 3144 (s, C-H stretch.), 1337 (s, CdC stretch.), 1028 (m, CdS stretching), 923 (w, C-S stretch.) [Cp2W(dmit)][PF6]. Cp2W(dmit) (10 mg) was oxidized by galvanostatic current in an electrolyte solution prepared by dissolving [Bu4N][PF6] (0.2 g) in 10 mL of freshly distilled CH2Cl2 at a constant current of 3 μA at room temperature. After one week, black needle-like crystals of the 1:1 salt were harvested from the anodic compartment of the electrochemical cell and washed with small amounts of freshly distilled CH2Cl2. Elem Anal. Calcd for C13H10F6PS5W (MW = 655.3666 g/mol): C, 23.83%; H, 1.54%. Found: C, 23.83; H, 1.44. IR (KBr, cm-1) 3093, 3122, 3138 (s, C-H stretch.), 1338 (s, CdC stretch.), 1030 (m, CdS stretch.), 923 (w, C-S stretching), 825, 842, 860 (s, PF6- f1u.)
Reinheimer et al. [Cp2W(dmit)][Br]. Cp2W(dmit) (22.1 mg) was oxidized by galvanostatic current in an electrolyte solution (0.5 M) prepared by dissolving [PPh4][Br] (0.2103 g, 0.501 mmol) in 10 mL of freshly distilled CH2Cl2 at a constant current of 5 μA at a temperature of -10 C. After one week, black needle-like crystals of the 1:1 salt were harvested from the anodic compartment of the electrochemical cell and washed with small amounts of freshly distilled CH2Cl2. Elem Anal. Calcd for C13H10BrS5W (MW = 590.3064 g/mol): C, 26.45%; H, 1.71%. Found: C, 26.29; H, 1.60. IR (KBr, cm-1) 3053, 3100 (s, C-H stretch.), 1348 (s, CdC stretch.), 1025 (m, CdS stretching), 911 (w, C-S stretch.). [Cp2W(dmit)][Au(CN)2]. Cp2W(dmit) (14.6 mg) was oxidized in an electrolyte solution (0.5 M) prepared by dissolving [n-Bu4N][Au(CN)2] (0.2464 g, 0.502 mmol) in 10 mL of freshly distilled CH2Cl2. Black needle-like single crystals of the 1:1 salt were grown using Pt electrodes at a constant current of 2 μA at 2 C, harvested after a period of one week, and washed with small amounts of freshly distilled CH2Cl2. Elem Anal. Calcd for C15H10AuN2S5W (MW = 759.4043 g/mol): C, 23.72%; H, 1.33%. Found: C, 23.60; H, 1.35. IR (KBr, cm-1) 3055, 3069, 3088, 3121, 3134 (s, C-H stretching), 2138 (s, CtN stretching), 1333, 1341, 1346 (s, CdC stretching), 1027 (w, CdS stretching), 923 (w, C-S stretching). DFT Calculations. The quantum chemical calculations of both the neutral complex and the [Cp2W(dmit)]þ• cation were carried out by using the Gaussian 3.0 program.41 The equilibrium geometry and vibrational transitions were evaluated by the B3LYP and B3PW91 functionals with the LanL2DZ basis set, in the gas phase. Molecular and vibrational properties of the [Cp2W(dmit)]þ• cation with different folding angles (θ) were also studied. In this case, the folding angle was fixed (θ = 0, 10, 20, 30), and then the geometry optimization process was performed. Raman Spectroscopy. Raman spectra of single crystals were recorded at room temperature in a backward scattering geometry using a Labram HR HORIBA Jobin Yvon spectrometer with a He-Ne laser (λexc = 632.8 nm). The spectra were measured for different orientations of the electrical vector of the exiting laser beam: either parallel or perpendicular to the direction corresponding to maximum intensity of the Raman bands assigned to CdC stretching vibration. The spectra were mainly recorded within the range of strongest vibrational features, i.e., over the wavenumber range 150-2000 cm-1. The power of the laser beam was about 0.1 mW, and the spectral resolution was 2 cm-1. Crystallography. Experimental data and refinement results are given in Table 9. Data were collected on an APEX II Bruker AXS diffractometer with Mo KR radiation (λ = 0.71073 A˚). Structures were solved by direct methods (SHELXS9742 or SIR92)43 and refined (SHELXL-97)42 by full-matrix least-squares methods as implemented in the WinGX software package.44 An empirical absorption (multiscan) correction was applied. (41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M.W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.05; Gaussian Inc.: Pittsburgh, PA, 2003. (42) Sheldrick, G. M. SHELX97, release 97-2; University of G€ottingen: G€ottingen, Germany, 1998
Article Hydrogen atoms were introduced at calculated positions (riding model) included in structure factor calculation but not refined.
Acknowledgment. We thank T. Guizouarn (Rennes) for the SQUID measurements, D. Łazarski (Pozna n) for help in Raman (43) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. (44) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.
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measurements, and B. Domercq (Nantes) for the first preparation of the [Cp2W(dmit)][PF6] salt. This work was supported by the French-Polish program Polonium (no. 20083YF), and the French Ministry of Foreign Affairs (Postdoctoral fellowship to E.W.R.). We also thank the Polish Ministry of Science and Higher Education for the research project in the years 2008-2010. Supporting Information Available: A crystallographic file in CIF format is provided. This material is available free of charge via the Internet at http://pubs.acs.org.
